JP2015141218A - Optical coupling structure, optical coupler, and optical module - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical coupling structure that efficiently converts a mode field between waveguides of different core numbers and can be manufactured at a low cost.SOLUTION: An optical coupling structure 300 includes: a multi-mode interferometer waveguide core 304 having a first input/output port 308 and second input/output ports 310A and 310B; and two taper optical waveguide cores 312A and 312B that are connected to respective second input/output ports and have a width gradually decreasing toward the outside until the lights received from the second input/output ports 310A and 310B are inter-coupled to provide light having one mode field.

Description

  The present invention relates to an optical coupling structure, and an optical coupler and an optical module using the optical coupling structure. More specifically, the present invention relates to an optical coupling structure that converts guided light between two optical waveguides having different numbers of cores. The present invention relates to an optical coupler and an optical module used.

  In an optical semiconductor device or the like used for an optical communication device, an optical coupling structure having a function of converting a field size of a light beam may be required. For example, the field size of the light beam output from the light source may be larger than the field size of the guided light of the optical waveguide coupled to the light source. (When the waveguide is a single mode, the field size of the guided light is synonymous with the mode field size.) In such a case, the field size of the output light from the light source is the field size of the guided light of the optical waveguide. An optical coupling structure is used to convert to. In particular, when an optical coupling structure is connected to a waveguide end and used for optical coupling with an external optical system (for example, a light source or an optical fiber), the optical coupling structure is a spot size converter (SSC, Spot-Size Converter). be called.

  Conventionally, in order to stably realize high-efficiency optical coupling with a light source, a single-core optical waveguide with a small mode field size is converted to a multi-core waveguide with a mode field size comparable to the output beam of the light source. It has been proposed to do. Just reducing the core size of a single-core waveguide has the effect of expanding the field size of the guided light. However, when the number of cores is converted and the field size of the guided light is expanded by using a multi-core waveguide. Compared with the case where the field size is increased with a single waveguide, the field size is less affected by variations in the core size, and high-efficiency optical coupling can be stably obtained.

  Patent Document 1 discloses an optical coupling structure that connects a single-core optical waveguide and a multi-core optical waveguide using a tapered structure. FIG. 1 shows such a conventional optical coupling structure 100. In FIG. 1, Gaussian-shaped figures such as the light 102A and the light 102B represent the field distribution (intensity distribution) of propagating light, and the interval between positions where the intensity is 1 / (equivalent to about 13.5%) of e The field size. Now, the light 102A incident on the optical coupling structure 100 is converted into the light 102B having a larger field size via the taper portion 104 that has spread. The light 102B is separated into a plurality of tapered portions (two tapered portions 106A and 106B in FIG. 1) and converted into light 102C and 102D. Each of the lights 102C and 102D gradually penetrates to the outside of the respective cores while propagating through the tapered portions 106A and 106B, and the overlap of the skirts of the fields is increased, so that one light having a mode field that is spread as a whole is obtained. 102E. The light 102E further spreads while propagating through the tapered portions 106A and 106B, and becomes light 102F having a field close to a larger Gaussian shape. In this way, the optical coupling structure 100 converts the field size of the guided light.

  Patent Documents 2 and 3 disclose optical coupling structures that connect a single-core optical waveguide and a multi-core optical waveguide using a speed-type directional coupler. FIG. 2 shows such a conventional optical coupling structure 200. Light 202A input to the optical coupling structure 200 is separated into light 202C and 202D input to the two tapered portions 206A and 206B via the velocity-type directional coupler 204, respectively. The light 202C and 202D gradually penetrates out of the core while propagating through the tapered portions 206A and 206B, respectively, and is combined into a single light 202E having an expanded mode field by increasing the overlap of the bottom of the field. Is done. The light 202E further spreads while propagating through the tapered portions 206A and 206B, and becomes light 202F having a field close to a larger Gaussian shape. The optical coupling structure 200 thus converts the field size of the guided light.

International Publication No. 2005/017588 Republished Patent International Publication No. 2011-036818 JP2013-140205A

  However, the conventional optical coupling structure has a problem that the optical loss is large and the manufacturing cost is high. First, in the optical coupling structure 100 shown in FIG. 1, the field from the light 102B to the light 102C and the light 102D is obtained during the optical coupling between the tapered portion 104 and the branched tapered portions 106A and 106B. A large optical loss occurs due to a drastic change in the distribution of light.

  In the optical coupling structure 200 shown in FIG. 2, if the width of the tip of the taper included in the velocity type directional coupler 204 is not sufficiently small, a large optical loss is caused. As a specific example, in the case of an optical waveguide having a silicon core with a height of 220 nm, if the wavelength of light in vacuum is 1.5 μm, the tip width of the taper must be 100 nm or less. Loss cannot be made. Therefore, if an attempt is made to use a lithography apparatus capable of patterning with a width of 100 nm for manufacturing, an electron beam lithography apparatus generally considered to have poor patterning efficiency is used, or an ArF immersion scanner that is more efficient but more expensive Is currently forced to use. As a result, as long as the conventional technique is used, an increase in manufacturing cost is inevitable regardless of which lithography apparatus is used.

When the optical coupling structure is used for optical coupling, the field size of the light 102F is determined according to the field size of the optical coupling destination. For example, if the mode field size is 3 μm, the tip width of the two-core waveguide is 130 μm, and the core interval is 1 μm.
Thus, according to the prior art, there is a problem that a core number conversion type optical coupling structure that exhibits sufficiently high optical coupling efficiency cannot be realized at low cost. Accordingly, it is an object to provide a core number conversion type optical coupling structure that can convert the field size of guided light with high efficiency and can be manufactured at low cost.

  In addition to the use as a spot size converter, the core number conversion type optical coupling structure may be used for highly efficient optical coupling between a plurality of core waveguides having different core numbers. The multi-core waveguide itself is used, for example, for the purpose of causing light distributed in the clad between the cores to efficiently act on the clad material. Conventionally, only optical coupling between a single-core waveguide and a plurality of core waveguides has been assumed, so that high-efficiency optical coupling between a multi-core waveguide and another multi-core waveguide having a different number of cores is possible. There was no practical optical coupling structure. Accordingly, it is another problem to provide an optical coupler structure that can convert guided light between a plurality of core waveguides having different numbers of cores with high efficiency.

  In an embodiment of the present invention, the core number conversion type optical coupling structure used for optical coupling of optical coupling structures or waveguides having different numbers of cores has M cores (M is an integer of 1 or more). An N-core waveguide having a core waveguide and N cores (N is an integer greater than M) is provided, and the maximum value of the lateral widths of the N cores of the N-core waveguide is the M-core waveguide. Less than M times N times the minimum value of the width of each of the M cores of the waveguide, and at least M and N input / output ports between the M core waveguide and the N core waveguide A multi-mode interferometer waveguide core, each core of the M-core waveguide is connected to M input / output ports of the multi-mode interferometer waveguide core, and each core of the N-core waveguide Are connected to the N input / output ports of the multimode interferometer waveguide core. Tapered waveguide core whose width gradually decreases toward the N-core waveguide between the core of the N-core waveguide and each input / output port of the multimode interferometer waveguide core to which the cores are connected The distance between the central axes of the tapered waveguide cores is constant between adjacent ones, or gradually increases or decreases as the distance from the multimode interferometer waveguide increases.

In the embodiment, the multi-mode interferometer waveguide core has a line-symmetric structure with respect to the central axis in the waveguide direction.
In an embodiment, M may be 1, in which case N may be 2.

In an embodiment, the width of each of the N cores of the N core waveguide is all less than the spacing between any two of the N cores.
An optical coupler according to an embodiment of the present invention includes an optical coupling structure as described above.

In an embodiment, the M core waveguide of the optical coupler may have an end face. In that case, the length of the M core waveguide may be zero.
In an embodiment, the N core waveguide may have an end face. In that case, the length of the N-core waveguide may be zero.

  An optical module according to an embodiment of the present invention may include the optical coupler and the light source as described above, and the light source may be optically coupled to the end face of the optical coupler.

It is the schematic of the optical coupling structure of a prior art. It is the schematic of the optical coupling structure of a prior art. 1 is a schematic view of an optical coupling structure according to an embodiment of the present invention. The operation | movement of the optical coupling structure of FIG. 3 is shown. 1 is a schematic view of an optical coupling structure according to an embodiment of the present invention. The operation | movement of the optical coupling structure of FIG. 5 is shown. 1 is a schematic view of an optical coupling structure according to an embodiment of the present invention. 1 is a schematic view of an optical coupling structure according to an embodiment of the present invention. 1 is a schematic view of an optical coupling structure according to an embodiment of the present invention. 1 is a schematic plan view of an optical module according to an embodiment of the present invention. 1 is a schematic plan view of an optical module according to an embodiment of the present invention.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 3 schematically illustrates an optical coupling structure 300 according to an embodiment of the present invention. In the optical coupling structure 300, light having a mode field of a single-mode waveguide (two-core waveguide) including two cores is input from a single-mode waveguide (one-core waveguide) including one core. Convert to and output. Usually, since the mode field of the two-core waveguide is larger than the mode field of the one-core waveguide, if this effect is used, the optical coupling with the external optical system can be achieved by providing the optical coupling structure 300 with an end face. It can also be used as a spot size converter for performing.

  The optical coupling structure 300 includes M (M is an integer of 1 or more, M = 1 in FIG. 3) first input / output ports 308 and N (N is an integer greater than M, N = 2 in FIG. 3). ) Second input / output ports 310A and 310B. In the multimode interferometer waveguide core 304, when the first input / output port 308 is on the input side, the second input / output ports 310A and 310B are on the output side, and the first input / output port 308 is on the output side. In this case, the second input / output ports 310A and 310B are on the input side.

  The optical coupling structure 300 further includes tapered optical waveguides 312A and 312B that are respectively connected to the second input / output ports 310A and 310B. As shown in FIG. 3, the tapered optical waveguides 312A and 312B gradually decrease in width toward the outside of the multimode interferometer waveguide core 304 (that is, along the direction away from the multimode interferometer waveguide core 304). Configured as follows.

  As shown in FIG. 3, the first input / output port 308 of the multimode interferometer waveguide core includes an M core waveguide 314 having M cores (M = 1, one core waveguide). It may be connected. Further, at the narrow ends of the tapered waveguide cores 312A and 312B, the cores 316A and 316B of the N-core waveguide 316 (N = 2, two-core waveguide) having N cores are respectively provided. It may be connected.

  A multi-core waveguide, including a two-core waveguide, is a single-mode waveguide that includes a plurality of adjacent cores. The electromagnetic field of guided light propagates through all the parallel cores in the same phase. It has only a guided mode. Here, a single mode waveguide refers to a waveguide that is single mode with respect to at least one of TE polarization and TM polarization. As a waveguide structure similar to such a multi-core waveguide, there is a structure in which the cores of a single waveguide are simply arranged in parallel, but in this case, the light propagating through each core The phases are not necessarily aligned and may change independently of each other. Qualitatively speaking, the difference in structure between a multi-core waveguide and a simple parallel arrangement of ordinary single-core waveguides is that the width and height of all cores contained in the former waveguide are It is smaller in each stage than any core of the parallel waveguide. By satisfying this condition sufficiently, only a single guided mode can exist.

  When the M core waveguide and the N core waveguide having a larger number of cores are both single mode waveguides, the width of each core of the M core waveguide and the width of each core of the N core waveguide are It is approximately inversely proportional. This is because the maximum value of the total cross-sectional area of the core in which the single mode is maintained is substantially constant. The ratio of the core widths is the largest in the case of the one-core waveguide and the two-core waveguide. The width of each core of the two-core waveguide is at most about half the width of the core of the one-core waveguide. . Therefore, quantitatively, when it is known that the M-core waveguide is a single-mode waveguide, the maximum value among the widths of the N cores of the N-core waveguide is the M-core waveguide. If the width of the N-core waveguide is designed to be not more than M times N of the minimum value of the width of each of the M cores, the N-core waveguide can also be a single-mode waveguide. .

The optical coupling structure 300 may be configured to have a line-symmetric structure with respect to the central axis of the multimode interferometer waveguide core 304 in the waveguide direction.
The distance between the central axes of the tapered optical waveguides 312A and 312B may be constant between the second input / output ports 310A and 310B and the cores 316A and 316B, or approaches the cores 316A and 316B. It may increase or decrease gradually.

  The field size of light guided along the optical waveguide core 314 connected to the first input / output port 308 is equal to the field size of light guided along the cores 316A and 316B of the multi-core waveguide 316. Is different. Therefore, the optical coupling structure 300 has an effect of converting the field size of the guided light at the same time as performing highly efficient optical coupling between a plurality of core waveguides including different numbers of cores.

  FIG. 4 illustrates the operation of the optical coupling structure 300 of FIG. Light 302 A propagating along the waveguide core 314 is input to the multimode interferometer waveguide core 304 at the first input / output port 308. The multimode interferometer waveguide core 304 separates the input light 302B into light 302C and 302D. The lights 302C and 302D are output to the tapered optical waveguide cores 312A and 312B via the second input / output ports 310A and 310B, respectively. The tapered optical waveguide cores 312A and 312B are configured such that the width gradually decreases toward the outside of the multimode interferometer waveguide core 304. Thus, the lights 302C and 302D ooze out of the waveguide core as they propagate along the tapered optical waveguide cores 312A and 312B, and the mode fields of these lights overlap as shown in 302E, and optically Join. The light 302E propagates further along the tapered optical waveguide cores 312A and 312B, and the mode field overlap is increased (ie, the coupling is increased), so that the light 302E is completely coupled into the light 302F having one mode field that is greatly expanded. (Ie, single mode). The light 302F propagates along the cores 316A and 316B of the multi-core waveguide 316 connected to the tapered optical waveguide cores 312A and 312B, respectively, while maintaining the expanded mode field. In this way, the optical coupling structure 300 can convert the light 302A having a small mode field into the light 302F having a large mode field in the process of performing optical coupling between a plurality of core waveguides having different core numbers.

  Here, the operation in the case where the light input from the single core waveguide including the core 314 is output to the plurality of optical waveguides 316 including the cores 316A and 316B has been described. However, for those skilled in the art, the optical coupling structure 300 allows the light input from the multi-core waveguide 316 including the optical waveguide cores 316A and 316B to be output to the single core waveguide including the optical waveguide core 314. It will be understood that the reverse operation is also possible. In this case, the light having a large mode field propagating along the optical waveguide cores 316A and 316B is separated into two independent lights via the tapered optical waveguide cores 312A and 312B, respectively. And 310B to the multimode interferometer waveguide core 304.

  Returning to inputting light from the waveguide core 314 side, the optical coupling structure 300 inputs the light 302B into the tapered waveguides of the cores 312A and 312B by using the multimode interferometer waveguide core 304. Before separation, they are separated in accordance with the field distribution of these tapered waveguides. Therefore, there is no mode field mismatch between the multimode interferometer waveguide core 304 and the tapered waveguide cores 312A and 312B, and the optical coupling efficiency is higher than that of the conventional optical coupling structure as shown in FIG. To realize.

  In addition, since the minimum dimension included in the optical coupling structure 300 is the width of the optical waveguide cores 316A and 316B (the width of the tips of the tapered optical waveguide cores 312A and 312B), for example, the optical coupling structure illustrated above is used. If it is a use, 130 nm should just be resolved. In the case of the conventional optical coupling structure as shown in FIG. 2, an ArF immersion scanner is necessary to resolve 100 nm of the tapered tip of the velocity type directional coupler. Can be patterned with an ArF dry scanner that is significantly less expensive than that. As described above, the optical coupling structure 300 is not wasted as it is forced to use an expensive exposure apparatus only for resolving the structure as in the prior art.

  Further, the multimode interferometer waveguide core 304 is less likely to deteriorate the branching function of the light beam even if it is shortened as compared with a velocity type directional coupler, and therefore can be made shorter. In the case of a speed-type directional coupler, if an attempt is made to shorten the speed-type directional coupler, the gap between each taper that is a component of the speed-type directional coupler must be narrowed to speed up the movement of light between the tapers. However, as described above, there is a limit to the resolution of the lithographic apparatus that can be used from the viewpoint of manufacturing cost, so there is a limit even if the gap is narrowed. On the other hand, in the case of the multimode interferometer waveguide core 304, since such a gap is not included in the structure, it cannot be shortened due to the limitation of lithography. In order to efficiently couple light in the conventional optical coupling structure, for example, it is necessary to use a speed type directional coupler having a length of about 80 μm. In contrast, in the optical coupling structure 300 of this embodiment, the multimode interferometer waveguide core 304 can be configured to have a length of, for example, about 5 μm in order to obtain the same optical branching characteristics. . For this reason, the optical coupling structure 300 of the present embodiment can be reduced in size as compared with the conventional optical coupling structure. Therefore, the optical coupling structure 300 of this embodiment can be incorporated in an optical module having a higher degree of integration than the conventional optical coupling structure.

  In the optical coupling structure 300 shown in FIG. 4, if most of the guided light field 302A propagating along the optical waveguide core 314 is confined in the optical waveguide core 314, the optical waveguide core 314 and The reflection loss and scattering loss at the connection point of the multimode interferometer waveguide core 304 are small enough to be ignored. Such a situation occurs when the width of the optical waveguide core 314 is sufficiently larger than the wavelength of light in the medium constituting the optical waveguide core 314.

  On the other hand, if the width of the optical waveguide core 314 is the same as or smaller than the wavelength of light in the medium constituting the waveguide core 314, the field of the guided light 302A from the optical waveguide core 314 oozes out. The reflection loss and the scattering loss at the connection point between the optical waveguide core 314 and the multimode interferometer waveguide core 304 become so large that they cannot be ignored. In that case, a tapered waveguide may be provided between the optical waveguide core 314 and the multimode interferometer waveguide core 304. Such an optical coupling structure will be described next.

  FIG. 5 schematically illustrates an optical coupling structure 500 according to an embodiment of the present invention. The optical coupling structure 500 converts guided light between multi-core waveguides having different numbers of cores. The optical coupling structure 500 includes M (M is an integer of 1 or more, M = 1 in FIG. 5) first input / output ports 508 and N (N is an integer greater than M, N = 2 in FIG. 5). ) Multi-mode interferometer waveguide core 504 having second input / output ports 510A and 510B. In the multimode interferometer waveguide core 504, when the first input / output port 508 functions as an input, the second input / output ports 510A and 510B function as an output, and the first input / output port 508 serves as an output. When functioning, the second input / output ports 510A and 510B function as inputs.

  The optical coupling structure 500 further includes a tapered optical waveguide core 518 connected to the first input / output port 508 and tapered optical waveguide cores 512A and 512B respectively connected to the second input / output ports 510A and 510B. As shown in FIG. 5, the tapered optical waveguide cores 512 </ b> A, 512 </ b> B, and 518 are configured so that the width gradually decreases toward the outside of the multimode interferometer waveguide core 504. In FIG. 5, the optical coupling structure 500 may include an optical waveguide core 514 having a certain width connected to the tapered optical waveguide core 518. The optical coupling structure 500 may also include a multi-core waveguide 516 including optical waveguide cores 516A and 516B having a certain width connected to the tapered optical waveguide cores 512A and 512B, respectively. The field size of light guided along the optical waveguide core 514 is different from the field size of light guided along the two-core waveguide 516 by the combination of the optical waveguide cores 516A and 516B. Therefore, the optical coupling structure 500 can convert the field size of the guided light.

  FIG. 6 illustrates the operation of the optical coupling structure 500 of FIG. Light 502A propagating along the optical waveguide core 514 is input to the tapered optical waveguide core 518. Since the width of the optical waveguide core 514 is small, the field of the light 502 </ b> A oozes outside the optical waveguide core 514. The tapered optical waveguide core 518 is configured to gradually increase in width toward the multimode interferometer waveguide core 504. For this reason, the field in which the light 502A has oozed out is absorbed into the tapered optical waveguide core 518 as it propagates along the tapered optical waveguide core 518, and most of the field enters the light 502B existing in the core. Converted. This light 502 B is input to the multimode interferometer waveguide core 504 at the first input / output port 508. Since the penetration of the light 502B to the outside of the core 518 is negligibly small, the reflection loss and the scattering loss generated at the connection point 508 between the tapered waveguide core 518 and the multimode interferometer waveguide core 504 are negligible. small. Multimode interferometer waveguide core 504 separates input light 502B into light 502C and 502D that are coupled to second input / output ports 510A and 510B, respectively. Lights 502C and 502D are output to tapered optical waveguide cores 512A and 512B via second input / output ports 510A and 510B, respectively. The tapered optical waveguide cores 512 </ b> A and 512 </ b> B are configured such that the width gradually decreases toward the outside of the multimode interferometer waveguide core 504. Therefore, the lights 502C and 502D gradually ooze out of the optical waveguides as they propagate through the tapered optical waveguides 512A and 512B, and the bottoms of the mode fields of these lights overlap as shown at 502E. The light 502E propagates further along the tapered optical waveguide cores 512A and 512B and is coupled into the light 502F having one mode field that is greatly expanded. The light 502F propagates while maintaining this extended field via the multi-core waveguide 516 including the optical waveguide cores 516A and 516B connected to the tapered optical waveguide cores 512A and 512B, respectively. In this way, the optical coupling structure 500 can convert light 502A having a small mode field into light 502F having a large mode field in the process of performing optical coupling between a plurality of core waveguides having different core numbers.

  Here, the operation when the light input from the optical waveguide core 514 is output to the optical waveguide cores 516A and 516B has been described. However, those skilled in the art will appreciate that the optical coupling structure 500 can also operate such that light input from the optical waveguide cores 516A and 516B is output to the optical waveguide core 514. In this case, incident light having a large field is converted into light having a smaller field.

  FIG. 7 schematically illustrates an optical coupling structure 700 according to an embodiment of the present invention. The optical coupling structure 700 has the same configuration as the optical coupling structure 500 shown in FIG. The optical coupling structure 700 includes M (M is an integer of 1 or more, M = 1 in FIG. 7) first input / output ports 708 and N (N is an integer greater than M, N = 2 in FIG. 7). ) Multi-mode interferometer waveguide core 704 having second input / output ports 710A and 710B. The optical coupling structure 700 further includes a tapered optical waveguide core 718 connected to the first input / output port 708 and tapered optical waveguide cores 712A and 712B respectively connected to the second input / output ports 710A and 710B. The optical coupling structure 700 may include an optical waveguide core 714 and a multi-core waveguide 716 including optical waveguide cores 716A and 716B. At least one of the tapered optical waveguide cores 712A and 712B may be configured to include one or more portions having a constant width. For example, as shown in FIG. 7, tapered optical waveguide cores 712A and 712B may be configured to include portions 720A and 720B, respectively, that have a constant width (ie, the width does not gradually decrease / increase). Although not shown, similarly, the tapered optical waveguide core 718 may have one or more portions having a certain width.

  FIG. 8 schematically illustrates an optical coupling structure 800 according to an embodiment of the present invention. As described above, the multimode interferometer waveguide core 804 includes M (M is an integer equal to or greater than 1) first input / output ports and N (N is an integer greater than M) second input / output. Has a port. In the optical coupling structure 300 illustrated in FIG. 3, M = 1 and N = 2. However, as illustrated in FIG. 8, for example, M = 1 and N = 3 may be used. In the optical coupling structure 800, the light that propagates along the optical waveguide core 814 and is input to the first input / output port 808 of the multimode interferometer waveguide core 804 is input to the second three input / output ports 810A and 810B. And output to 810C. The output light propagates along the tapered optical waveguide cores 812A, 812B and 812C, respectively, and is combined into light having one mode field, and then the three cores by the combination of the optical waveguide cores 816A, 816B and 816C. The light is guided along the waveguide 816 while maintaining its mode field.

  The optical waveguide cores 816A, 816B, and 816C are N-core waveguides having N (N = 3) cores, that is, cores of three-core waveguides 816. When the three-core waveguide 816 is a single-mode waveguide, in many cases, the lateral width of any of the optical waveguide cores 816A, 816B, and 816C is equal to that of the optical waveguide cores 816A, 816B, and 816C. It is smaller than the interval between the cores of any two of the optical waveguides. This is because, if the light field oozes out from the individual waveguide cores is not sufficiently large, coupling between the fields does not occur sufficiently, and a single mode is not obtained.

  FIG. 9 schematically illustrates an optical coupling structure 900 according to an embodiment of the present invention. The multimode interferometer waveguide core 904 has M (M is an integer equal to or greater than 1) first input / output ports and N (N is an integer greater than M) second input / output ports. In the optical coupling structure 500 shown in FIG. 5, M = 1 and N = 2, but as shown in FIG. 9, for example, M = 2 and N = 4 may be used. In the optical coupling structure 900, the light guided along the optical waveguide cores 914A and 914B of the two-core waveguide 914 is divided into two optical fields in the process of propagating through the tapered optical waveguide cores 918A and 918B. The signals are input to the first input / output ports 908A and 908B of the multimode interferometer waveguide core 904. As described above, when a core of a multi-core waveguide including two or more cores is connected to the multi-mode interferometer waveguide core, the multi-core waveguide and the multi-mode interferometer are guided through the tapered waveguide core. The light field can be continuously changed (that is, adiabatic) between the input ports of the waveguide core, and light loss due to reflection and scattering can be reduced. The light that enters the multi-core waveguide from the input / output ports 908A and 908B is separated into four second input / output ports 910A, 910B, 910C, and 910D and output. The output light propagates along the tapered optical waveguide cores 912A, 912B, 912C, and 912D, respectively, and is combined into light having one mode field, and then the optical waveguide cores 916A, 916B, 916C, and 916D The four-core waveguide 916 by combination is guided while maintaining this mode field.

  Since the two-core waveguide 914 and the four-core waveguide 916 are both single mode waveguides, the maximum width of the optical waveguide cores 916A, 916B, 916C, and 916D is equal to the width of the optical waveguide cores 914A and 914B. It may be less than half of the minimum value.

An optical coupler (optical coupling structure) can be configured using the optical coupling structure of the embodiment of the present invention. FIG. 10 shows a schematic plan view of an exemplary optical module 1030 constructed by using the optical coupling structure according to the embodiment of the present invention as shown in FIG. The optical module 1030 is formed on, for example, an SOI (Silicon on Insulator) substrate having a structure in which a silicon dioxide (SiO ₂ ) layer (referred to as a buried oxide film) and a surface Si layer are stacked on the Si substrate. It may be formed. In this case, the multimode interferometer waveguide core 1004, the tapered optical waveguide cores 1018, 1012A and 1012B, and the optical waveguide core 1014 can be configured by processing the surface Si layer. The buried oxide film serves as a lower cladding. Silicon dioxide or another dielectric film may be further laminated as the upper clad. In this embodiment, the multimode interferometer waveguide core 1004 has a first input / output port and a second two input / output ports. The first input / output port is connected to the tapered optical waveguide core 1018, and the second input / output port is connected to the tapered optical waveguide cores 1012A and 1012B.

  In this embodiment, the first input / output port functions as an input, and the second input / output port functions as an output. The optical coupler 1000 couples the light guided along the optical waveguide core 1014 into the optical waveguide 1022 having different size mode fields. The optical waveguide 1022 has a core 1024. For example, the optical waveguide 1022 may be an optical fiber having a core 1024 having a circular cross section. The tapered optical waveguide cores 1012A and 1012B may have a uniform end surface at the narrowest end portion. Alternatively, a core of a multi-core waveguide (two-core waveguide) for maintaining a mode field may be further connected to the tapered optical waveguide cores 1012A and 1012B, and the core may have a uniform end surface.

  The light guided along the optical waveguide core 1014 is input to the multimode interferometer waveguide core 1004 via the tapered optical waveguide core 1018. The light is split into two by a multimode interferometer waveguide core 1004 and output to tapered optical waveguide cores 1012A and 1012B. The tapered optical waveguide cores 1012A and 1012B have a width that gradually decreases along the direction away from the multimode interferometer waveguide core 1004. Light propagating along the tapered optical waveguide cores 1012A and 1012B is coupled into light having one mode field in the process of reaching the end face of the optical module 1030. The mode field of the combined light is larger than the mode field of the light guided along the optical waveguide core 1014. Thus, the optical coupler 1000 can convert light having a small mode field into light having a larger mode field and couple it into an optical waveguide 1022 having a wide core 1024.

  The light output from the tapered optical waveguide cores 1012A and 1012B may have an elliptical mode field extending vertically or horizontally. In this case, as shown in FIG. 10, elliptical mode field light may be shaped into circular mode field light corresponding to the core of the optical waveguide 1022 by using a lens 1020. Of course, if the field of light output from the tapered optical waveguide cores 1012A and 1012B is comparable to the mode field of the optical waveguide 1022, such a lens is unnecessary, and low-loss optical coupling is possible by butt connection. It is.

  FIG. 11 shows a schematic plan view of an exemplary optical module 1130 constructed by using the optical coupling structure according to the embodiment of the present invention as shown in FIG. 5 as the optical coupler 1100. The optical module 1130 may be formed on an SOI substrate, for example. In this case, the multimode interferometer waveguide core 1104, the tapered optical waveguide cores 1118, 1112A and 1112B, and the optical waveguide core 1114 can be formed by processing the surface Si layer on the SOI substrate.

  In this embodiment, a light source 1126 is mounted on the optical module 1130. In the example of FIG. 11, the light source 1126 is a semiconductor laser and has an active region 1128. Further, the tapered optical waveguide cores 1112A and 1112B have uniform end surfaces at the narrowest end portions thereof. In this case, the light source 1126 may be optically coupled to the end surfaces by being disposed so as to abut against these end surfaces. Alternatively, a plurality of core waveguides (two-core waveguides) for maintaining the mode field may be further connected to the tapered optical waveguide cores 1112A and 1112B, and the cores may have uniform end faces. In the optical module 1130, the multimode interferometer waveguide core 1104 has one first input / output port and two second input / output ports, and the first input / output port functions as an output, The input / output port functions as an input. The optical coupler 1100 converts the field of light output from the active region 1128 of the semiconductor laser 1126 into a field suitable for being guided along the optical waveguide core 1114.

  The light output from the semiconductor laser 1126 is input to a multi-core waveguide (two-core waveguide) connected to the narrowest ends of the tapered optical waveguide cores 1112A and 1112B. As shown in FIG. 11, the length of the multi-core waveguide is zero, that is, the multi-core waveguide may not be provided. However, if there is a finite length, the position of the end face varies slightly due to processing accuracy. Even so, a stable end face shape can be obtained. Light input to the narrow ends of the tapered optical waveguide cores 1112A and 1112B is split into two lights having separate mode fields as they propagate toward the wide ends. These lights are respectively input to two second input / output ports of the multimode interferometer waveguide core 1104, are combined into one light by the multimode interferometer waveguide core 1104, and are tapered from the first input / output port. It is output to the optical waveguide core 1118. A mode field of light input to the tapered optical waveguide core 1118 is converted into light having a mode field having a size suitable for being guided along the optical waveguide core 1114 via the tapered waveguide core 1118. Is done.

  As described above, in the optical module 1130, by using the optical coupler 1100 according to the embodiment of the present invention, the light output from the semiconductor laser having the large mode field is transmitted to the optical waveguide core 1114 having the small mode field. It can be converted into light guided along.

  Although the invention has been described with reference to particular embodiments, the embodiments described herein are not intended to be construed in a limiting sense, and are intended to illustrate the invention. It is intended. It will be apparent to those skilled in the art that other alternative embodiments can be practiced without departing from the scope of the invention.

100, 200, 300, 500, 700, 800, 900 Optical coupling structure 102A, 102B, 102C, 102D, 102E, 102F, 202A, 202C, 202D, 202E, 202F, 302A, 302B, 302C, 302D, 302E, 302F, 502A, 502B, 502C, 502D, 502E, 502F Propagated light field distribution 104, 106A, 106B, 206A, 206B Taper 204 Speed type directional coupler 304, 504, 704, 804, 904, 1004, 1104 Multimode interference Waveguide core 308, 508, 708, 808, 908A, 908B First input / output port 310A, 310B, 510A, 510B, 710A, 710B, 810A, 810B, 810C, 910A, 910B, 910 910D Second input / output port 312A, 312B, 512A, 512B, 518, 712A, 712B, 718, 812A, 812B, 812C, 912A, 912B, 912C, 912D, 918A, 918B, 1012A, 1012B, 1018, 1112A, 1112B, 1118 Tapered optical waveguide core 314, 316A, 316B, 514, 516A, 516B, 714, 716A, 716B, 814, 816A, 816B, 816C, 914A, 914B, 916A, 916B, 916C, 916D, 914, 1114, 1114 Cores 316, 516, 716, 816, 914, 916 Multiple core waveguides 720A, 720B Parts having a constant width 1000, 1100 Optical coupler 1020 Lens 1022 Optical waveguide 1024 A 1030, 1130 Optical module 1126 Light source 1128 Active region

Claims (11)

A core number conversion type optical coupling structure that optically couples waveguides having different core numbers,
An M core waveguide having M cores (M is an integer equal to or greater than 1) and an N core waveguide having N cores (N is an integer greater than M), The maximum value of the lateral widths of the cores is equal to or less than M times N times the minimum value of the lateral widths of the M cores of the M core waveguide, and the M core waveguide and the N core waveguide. When,
A multimode interferometer waveguide core having at least M and N input / output ports disposed between the M core waveguide and the N core waveguide, wherein each core of the M core waveguide includes: The multimode interferometer waveguide core is connected to M input / output ports, and each core of the N core waveguide is connected to N input / output ports of the multimode interferometer waveguide core. A mode interferometer waveguide core;
A width toward the N-core waveguide is arranged between each core of the N-core waveguide and each input / output port of the multimode interferometer waveguide core to which the cores are connected. The taper waveguide core gradually decreases, and the interval between the central axes of the taper waveguide cores is constant between adjacent ones, or increases or decreases gradually as the distance from the multimode interferometer waveguide increases. An optical coupling structure comprising: a tapered waveguide core.   The optical coupling structure according to claim 1, wherein at least one of the tapered optical waveguides includes one or more portions having a constant width.   3. The optical coupling structure according to claim 1, wherein the optical coupling structure has an axisymmetric structure with respect to a central axis in a waveguide direction of the multimode interferometer waveguide core.   The optical coupling structure according to claim 1, wherein M is 1. 5.   The optical coupling structure according to claim 4, wherein N is two.   The width of each of the N cores of the N core waveguide is all smaller than any two spacings of the N cores, according to any one of claims 1-5. Optical coupling structure.   The optical coupler including the optical coupling structure according to claim 1, wherein the M core waveguide includes an end face.   The optical coupler according to claim 7, wherein the length of the M core waveguide is zero.   The optical coupler including the optical coupling structure according to claim 1, wherein the N core waveguide includes an end face.   The optical coupler according to claim 9, wherein the length of the N-core waveguide is zero. An optical coupler according to claim 7 or 10,
An optical module comprising: a light source; and the light source is optically coupled to the end face of the optical coupler.

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